BUG/MINOR: promex: Detach promex from the server on error dump its metrics dump

BUG/MINOR: promex: Detach promex from the server on error dump its metrics dump

If an error occurres during the dump of a metric for a server, we must take
care to detach promex from the watcher list for this server. It must be
performed explicitly because on error, the applet state (st1) is changed, so
it is not possible to detach it during the applet release stage.

This patch must be backported with b4f64c0ab ("BUG/MEDIUM: promex: server
iteration may rely on stale server") as far as 3.0. On older versions, 2.8
and 2.6, the watcher_detach() line must be changed by "srv_drop(ctx->p[1])".